This invention relates to a color picture tube of the type having a low expansion color-selection electrode or shadow mask attached to a frame which is suspended in spaced-relation to a cathodoluminescent screen comprising a mosaic of blue-emitting, green-emitting and red-emitting phosphor elements. More particularly, the invention relates to a support structure for such a tube which includes temperature compensation means which compensates for expansion of the tube envelope and faceplate panel caused by an increase in ambient temperature and the impingement of electron beams upon the color-selection electrode and screen.
U.S. Pat. No. 3,803,436 issued to Morrell on Apr. 9, 1974, discloses a plurality of shadow mask mounting assemblies. Each of the assemblies comprises a bimetal member attached to a shadow mask frame with a spring which interconnects the bimetal member to a stud embedded in the glass faceplate panel of the tube. The frame and shadow mask are conventional, i.e., they are preferably made of cold-rolled steel having a positive coefficient of thermal expansion, such that the frame and mask expand as their temperature increases. During the operation of the tube, the shadow mask intercepts about 80 percent or more of the electron beams directed from an electron gun toward the screen of the tube. The intercepted beams strike the mask and increase the temperature of the mask and frame causing them to expand. The outward expansion of the mask in relation to the screen would, without correction, cause misregister of the portions of the beams transmitted through the mask with the color elements of the screen. Correction or compensation is provided by the bimetal members attached to the frame, which react to the temperature increase of the frame and mask, and provide a compensating movement of the mask and frame toward the screen, thereby maintaining the desired mask-to-screen register.
U.S. Pat. No. 3,808,493 issued to Kawamura et al. on Apr. 30, 1974, discloses a shadow mask formed of Invar which has a coefficient of thermal expansion less than that of the cold-rolled steel mask described in the aforementioned Morrell patent. The Kawamura et al. patent discloses that the expansion of the Invar mask is negligible; however, the patent does not consider the heating effect on the glass envelope and faceplate panel of the ambient atmosphere and of the heat radiated from the shadow mask or from the impingement of electron beams on the faceplate panel of the tube.
The ambient temperature for color picture tube operation may be expected to vary from about 10.degree. C. to about 45.degree. C. for a temperature difference, .DELTA., of 35.degree. C. Also, during tube operation, as the electron beams strike the shadow mask and faceplate panel raising the temperature of the shadow mask to about 35.degree. C. above the ambient temperature, the faceplate panel temperature also will increase by about 7.degree. C. In the worst case, the faceplate will experience a temperature change of about 42.degree. C. (.DELTA.(35.degree. C.)+7.degree. C.). The coefficient of thermal expansion of the glass used for the faceplate panel is about 99.times.10.sup.-7 /.degree. C. so that for a 42.degree. C. change in faceplate temperature, the glass will expand more than about 3 micrometers (.mu.m) per centimeter. For a tube having a 25 inch diagonal faceplate, there would be about a 75 .mu.m screen displacement at the ends of the major axis which would be sufficient to produce misregister of the transmitted beams with the phosphor elements. Comparable inward beam displacement results from the funnel expansion which displaces the yoke and increases the screen to yoke deflection center, producing an additional 75 .mu.m misregister.